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Currently, NIFTy allows to construct a SmoothingOperator like this:

import nifty as ift
op=ift.SmoothingOperator(ift.RGSpace(10),sigma=1.,log_distances=True)

I do not think this makes sense, as the RGSace is periodic and there are no absolute distances. This combination of parameters also leads to overflows because the code in FFTSmoothingOperator tries to compute log(0.).

Therefore I propose to ensure that log_distance==False whenever an FFTSmoothingOperator is constructed and raise an exception otherwise.